The invention relates to a method of tuning a summing network of a base station, the summing network comprising: filter means with means for receiving signals supplied by radio transmitters of the base station, for filtering the signals, and for forwarding the filtered signals; and a summing member with in-connectors for receiving and combining the signals supplied by the filters, and an out-connector for supplying the combined signals to antenna means of the base station. The invention also relates to a base station of a radio system comprising: at least two transmitters; filter means, each one of which is connected to the output of the corresponding transmitter so as to filter the signals obtained from the output of the transmitter concerned and to supply the filtered signals further; and a summing member with in-connectors, each one of which is connected to the output of the corresponding filter so as to receive and combine the filtered signals, and an out-connector for supplying the combined signals to antenna means of the base station, at least one of the connectors of the summing member being coaxial and comprising an elongated rod-like inner conductor and a substantially tubular outer conductor surrounding the rod-like inner conductor. The invention further relates to a summing member comprising: in-connectors for receiving and combining at least two different RF signals, and an out-connector for supplying the combined signals further, whereby at least one of the connectors is coaxial and comprises an elongated rod-like inner conductor and a substantially tubular outer conductor surrounding the rod-like inner conductor.
The invention particularly relates to a summing network of combiner filters of a base station in a cellular radio system. A combiner filter is a narrowband band-pass filter in resonance with (tuned to) the carrier frequency of a transmitter connected to it. The adjustment range of the filters is usually 2-10% of the medium frequency. The signals obtained from the outputs of the combiner filters are summed by the summing network of the base station and supplied to the antenna of the base station. The summing network usually comprises a coaxial cable which leads to the antenna of the base station and to which the combiner filters are connected. In order that a maximum amount of the transmission power of the transmitters could transfer to the antenna, the summing network must be tuned to the frequency channels used by the transmitters of the base station. To be exact, the summing network is tuned at only one frequency, but with movement away from the optimum frequency, the mismatch will not rise strikingly at first. In the base stations of cellular radio systems, the summing network can thus usually be used at a frequency band with a width of about 1 to 3% of the medium frequency of the frequency band.
Tuning of previously known summing networks is based on the use of transmission lines of a precise, defined length, proportional to the wavelength. This sets high requirements to the cabling of a summing network, since the transmission lines must be of exactly the correct length so as to optimize the summing network to the correct frequency. As the operating frequency grows, the wavelength is reduced, and so is the length of the summing network. The requirements of tolerance set to the manufacture of branches of a summing network thereby increase so that it becomes impossible--or at least very expensive--to manufacture cables of a precise length. Also, as automatically (remote control) adjustable combiner filters have become more common, a need has arisen to change the tuning of the summing network in a simple and quick manner. The useful frequency band of the summing network is in practice so narrow that the frequency channels of the transmitters of the base station can hardly be changed at all if the tuning of the summing network is not adjusted as well. The previously known solution in which an installer goes to a base station site to replace the cabling of the summing network with cabling designed for a new frequency band is naturally too expensive and takes time.